The Football Association has been warned by Multiplex that it will be forced to foot some of the soaring costs of Wembley stadium. The threat emerged yesterday as reports suggested the firm's budget overruns on its £445m design-and-construction contract could stretch beyond £150m.

The magazine Construction News will say today that the steel cost for the project will be some £90m more than was originally budgeted. The original contractor, Cleveland Bridge, left the project after a series of disputes that are now the subject of a court case.

Its replacement, Hollandia, brought to the project in July 2004, would only take the deal on the basis that it charged for the cost of the materials plus labour and expertise, rather than enter into a fixed-price deal. The project has therefore suffered for the high price of steel over recent years. Figures due to be revealed over the next few weeks could reveal that Multiplex's total overrun on the project will be as high as £150m.

Some believe the overrun could be even greater. The magazine Construction News quoted one project source as saying: "Multiplex has to be looking at a £200m hit on Wembley." If the report is correct, the total cost of the Wembley project would approach £1bn.

Multiplex's relationship with WNSL, the FA subsidiary which runs the Wembley project, is likely to be tested in coming weeks. The stadium was originally due to be delivered on January 31, and penalty charges of almost £1m a week are due to be levied about now. Those payments are the subject of negotiations between the two parties. Multiplex is also suggesting it may try to claw back some of the cost overruns from WNSL, despite the fact it signed a fixed-price contract. "Not all these things are blown out because of a few problems with steel," a Multiplex source told the Guardian. "The FA are being optimistic if they think they've got no ownership of this. WNSL and the FA won't walk away Scot free. There is still information required for us to complete the work. And there is still information they're hoping to change."

That is believed to be a reference to design changes that have been brought into the project. Multiplex has had to take on the costs associated with accelerating the works programme on the Olympic Pedway after access to this area of the site was granted several weeks late following a dispute between WNSL and the landowner Quintain Estates. "In terms of the design changes, it's an ongoing process and is still a material risk [to timely completion] in our opinion," said the project director Ashley Muldoon on Monday. "[An] important aspect is how we dovetail with WNSL; some of the operational issues they may seek to modify. If there's a change and we think it will have a negative effect, we will tell them."

If the "70% chance" of the FA Cup final going ahead on May 13 that Multiplex suggested on Monday fails to bear fruit, the FA will be severely out of pocket. In 2001 WNSL stated its expectation of raising £200,000 from each concert staged at Wembley: two Bon Jovi dates follow shortly after the FA Cup final, and they would also be under threat with delays.